Known in the art are single crystals of yttrium-aluminium garnet Y.sub.3 Al.sub.5 O.sub.12 containing cobalt as a dyeing additive in an amount of from 0.01 to 3% by weight. Depending on the amount of the dyeing additive the crystals have a blue, green-blue or green colour (cf. CSSR Pat. No. 152909 Int. Cl. B 01 j 17/02, published 08.12.71).
Also known are single crystals of yttrium-aluminium garnet Y.sub.3 Al.sub.5 O.sub.12 of a green colour containing, as the dyeing additive, vanadium in the amount of 0.01% by weight (cf. CSSR Pat. No. 153870, Int. Cl. B 01 j 17/02, published 25.10.71).
To produce Y.sub.3 Al.sub.5 O.sub.12 crystals of a deep green colour with a slight sky-blue and emerald shade, chromium oxides are introduced into the starting charge in an amount of 0.11 to 0.27% by weight, cobalt oxide--in an amount of 0.21-1.38% by weight, iron oxide in an amount of 0.20 to 1.33% by weight and neodymium oxide--in an amount of 0.05 to 0.30% by weight (cf. U.S. Pat. No. 3,761,292 Int. Cl. C 04 B 35/00, published 25.09.73).
These materials contain, as dyeing additives, chromium, vanadium, cobalt, iron. Oxides of these dyeing additives have relatively high values of vapour tension which causes difficulties in the process of the production of uniform deep-coloured single crystals, especially under high vacuum conditions. This factor limits the opportunities for a large-scale production of these materials for jewelry applications.
Furthermore, formulations of these materials do not provide for the possibility of controlling the brightness of the material dyeing into a definite colour, smooth colour variation within a broad colour range, as well as obtaining a violet colour which is of interest for simulation of, for example, amethyst in jewelry articles.